The poll was published as Nato announced that six Nato troops, including three Americans and a French officer were killed in separate incidents, making it the deadliest day for the international force in more than two months.

The latest casualties come amid warnings that the bloodshed will increase as more foreign troops are deployed. President Barack Obama is sending an additional 30,000 US soldiers, bringing the total US deployment to 98,000.

General Stanley McChrystal, the top US commander in Afghanistan and the architect of the troop surge, said he believed the reinforcements were starting to turn the tide against the Taliban.

McChrystal said he was encouraged by a recent meeting he held in a former Taliban stronghold. "When I sit in an area that the Taliban controlled only seven months ago and now you meet with a shura [a traditional meeting] of elders and they describe with considerable optimism the future, you sense the tide is turning," he said.

His comments echoed remarks the head of the British army, General Sir David Richards, made yesterday. Richards told the BBC that the mission would benefit from more soldiers, but this would be another "tough year".

Last year was the most costly for UK forces since the Falklands war in 1982, with 108 troops killed in Afghanistan. Richards said he expected the level of British casualties to start decreasing by the end of this year.

"I personally anticipate as we get this business of mass right ‑ the numbers of boots on the ground, a result of allied enhancements and a growth in the Afghan army and police ‑ that I would see a diminishing level of casualties from the end of this year," he told BBC Radio 5 Live.

Support for Nato troops is up slightly from 59% in 2009 to 62% now, according to the BBC poll of more than 1,500 Afghans. Only 8% of those polled thought attacks on foreign troops could be justified, compared with 25% last year.

But the survey also uncovered serious concerns about corruption within the government and police, with 95% of those questioned saying it was a problem in their area and 44% saying things were worse than a year ago. Only 39% believed last year's election was conducted honestly and 59% thought the counting of votes was fraudulent. The Taliban remain very unpopular: 69% of Afghans think they pose the greatest danger to the country's future.